Various methods for preparing carboxylic acids by fermentation processes are known from the prior art, e.g. for succinic acid, lactic acid or citric acid. For optimal process conditions in the fermenter, the pH of the fermentation broth is adjusted by addition of a base (e.g. ammonium hydroxide, ammonium bicarbonate, sodium hydroxide, calcium hydroxide, etc.). Depending on the pH, this leads to the formation of a carboxylic acid salt, e.g. diammonium succinate in the case of neutralization of succinic acid with an ammonium base, or a mixture of carboxylic acid and salt.
The conversion of carboxylic acid salts to the free carboxylic acid again, e.g. by electrodialysis, is known from the prior art. However, such methods are linked to high energy consumption and tend to lead to fouling of the surface of the membranes, whereby the service life of the membranes is severely limited.
Other processes involve an acidification step, in which the carboxylic acid is isolated by the addition of a strong acid, while the carboxylic acid salt arising in this case remains in solution (in the case of ammonium sulfate) or in the suspension (in the case of calcium sulfate, the so-called gypsum process). The acidification of diammonium succinate with sulfuric acid thus leads to the formation of ammonium sulfate, a valuable fertilizer.
In order to achieve a separation of the carboxylic acid from the salt, a filtration is carried out in the “gypsum process”. It is, however, disadvantageous that the gypsum formed in this case is a waste product and cannot be reutilized.
A further variant for the separation of carboxylic acid and salt is based on a chromatographic separation, e.g. continuous chromatography (SMB, “simulated moving bed”). Due to the high apparatus costs for the chromatographic unit and the high consumption of water as eluent for an efficient separation of salt and acid, there are disadvantages here in terms of process economy.
Owing to the long residence times at high temperature which are required for the evaporation of the water, it is also disadvantageous that a discoloration of the salt generally occurs, which is a result of reactions of the remaining amino acids and the sugar in the salt-containing raffinate stream during the chromatography.
Further steps are required for the recovery of the carboxylic acid or carboxylic anhydrides at the desired purity. Established technologies for this purpose include, for example, ion exchange, nanofiltration, reverse osmosis, extraction, evaporation, distillation, crystallization or recrystallization. Here, the higher the purity requirements for the carboxylic acid, the greater however the complexity linked to the purification and the losses with regard to the yield.
In the case of succinic acid, the most important application here can be seen in the preparation of 1,4-butanediol (BDO), tetrahydrofuran (THF) and γ-butyrolactone (GBL). The last is the starting material for the preparation of 2-pyrrolidone.
BDO, THF and GBL may be prepared by an esterification and hydrogenation process, the DAVY process, starting from maleic anhydride. An intermediate product in this process is dimethyl succinate (DMS). Since this is prepared by an esterification of succinic acid, DMS could be fed into a conventional hydrogenation process for the preparation of BDO, THF or GBL.
In order to be competitive with comparable conventional starting materials, it is necessary to make the process of isolation and purification of the fermented starting materials as efficient as possible. In the case of succinic acid and derivatives thereof, the purification and crystallization of the succinic acid and of the succinic anhydride and the subsequent steps of the dissolution in methanol, esterification and hydrogenation are regarded as weaknesses with regard to efficiency due to the high number of process steps, the energy consumption and the many phase transitions.
Furthermore, it is known that the evaporative crystallization of carboxylic acids, such as succinic acid, is a sensitive process which influences the achievable purity of the crystals and the amount of impurities due to inclusions or sorption effects. It may therefore be necessary to involve a crystallization/recrystallization in order to reduce the impurities to an acceptable degree for the subsequent esterification.
In addition to the desired carboxylic acid, further carboxylic acids are also typically formed as by-products in fermentation processes, which can only be removed with great difficulty by the separation methods mentioned above. The production of succinic acid by fermentation leads at the same time to the formation of, for example, inter alia, acetic acid, lactic acid, fumaric acid and maleic acid as by-products. Depending on the specification of the succinic acid for the esterification and hydrogenation steps for the formation of DMS or even for the subsequent preparation of biopolymers such as polybutylene succinate (PBS), the accumulation of these further carboxylic acids as by-products can lead to formation of undesired alcohols or esters.
A major obstacle for biotechnological processes is the amount of water used. This relates to the energy efficient separation of the product, the large amounts of waste water generated and the requirement for catalytic reactions in an aqueous environment.
To provide efficient processes, depending on the end product, the dissolution behavior of the target components must be taken into account and catalytic processes have to be adjusted.
Therefore, a need exists for a method for preparing and isolating carboxylic esters that ensures a high product purity and minimizes the technical complexity of the individual method steps.